Torque-down and head gasket problems
A regular visitor to the message board, and something I am asked frequently about in my other incarnations for other technical media. Generally when something has gone badly wrong! Many folks have just as many ways of dealing with the following subjects - all can and will claim their way has worked for them for the proverbial millennia - so a few words as a basic reference and guide as I see them; we all know there are always exceptions to the rules. This is to help those floundering in an unknown sea of techno-babble and 'magical' cures…
First a brief word on head gaskets. If it is not a genuine Rover or Payen gasket, don’t use it; at least half the problems presented to me are caused by cheapo head gaskets. If it’s a copper-composite gasket, make sure it’s got a slate-grey coloured ‘filling’ and a silver spray-paint-like coloured underside. If it’s a shiny tin/steel colour (looks like clean bare metal), forget it. If it's got a copper top surface and a black under-side (looks like painted metal), forget it. If it’s a dark charcoal-coloured-looking thing, chances are it’s genuine Rover. And the small-bore, strange, silver-grey-coloured things that look more like exhaust manifold gasket material than head gasket material supplied in Rover/Unipart head gasket kits are OK for standard motors. Anything else - leave well alone!
OK. Basic brief on head gaskets over – head studs and torque settings. Now, I've heard all SORTS of weird stuff about how folk torque their head studs down. VERY weird. I don’t understand any of this as it’s plainly laid out in black and white print in a million manuals. All you have to do is torque the standard head studs down to 42-45lb ft (a generous lee-way there I feel), DRY. That’s no lubrication on nuts, washers, or studs. ALL torque settings in the manuals are quoted with the relevant threads DRY. And that's the way they're done at the factory.
‘Ah-ha’, many have said (or are saying now),’but I’ve got a set of super-fandangly-mega-maglorious head studs in my soopa-sonicly modified thirteen-eighty-fourteen-thirty-fourteen-hundred-offset-over-wangled mota!’ - assuming something more brutal in the way of torque-settings will be needed to contain the rampant mega-horsepower this incredible unit will no doubt produce. Or why the standard set-up is finding it impossible to deal with all that 'energy'. Well, the vendor should have issued you with the relevant instructions on how to use them. Or perhaps it's a case of ‘I know how to fit these!’ Believe me when I say - once I have enlightened these folks, the faces said it all.
When using almost any engine cylinder head stud/nut combination – be it standard or an up-graded after-market type – there are certain points that must be remembered.
1 – DO NOT use any sort of thread lock compound. Yes, some folk did. No, I couldn’t believe it either. I have to say it’s a new one on me. Big-end bolts/nuts? Yep. Heard that one. Mains bolts/studs/nuts? Yep. Heard that one too. But head studs/nuts? Nope. Never. The excuses (sorry, reasons) were many – but the ‘well I couldn’t afford new ones, and I thought the old ones might have worn threads’ was the most common. Unless you use the specific thread-locking compound that is made just for the application you are using it for - it will detract markedly from the actual torque setting - some by as much as 30%. This is because the compound starts to 'set' with the friction generated when doing a nut/bolt up. Plus it can get under the head of the nut/bolt, set when torquing up, but will 'powder' from vibration/pressure when the engine is used. A small gap is then caused, and the torque setting/clamping pressure is relaxed/reduced. Not good.
2 – If standard spec studs/nuts are used, do them up with NO lubrication to 45lb ft. If you must oil the threads, do them up to 42lb ft, or if using a moly-based oil, 40lb ft.
3 – If using either Mini Spares up-graded stud/nut set OR ARP ones, use a lubricant – and preferably a moly-based one – and torque ‘em up to 45lb ft.
Why? The main reason (there are several) is because over-torquing will almost certainly distort the gasket to the point where failure is likely to occur. The most likely areas are the fire-rings that seal the head face (chambers) to block face (bores) off. Particularly between the bores where the steel fillet inserts are in non-copper/composite gaskets, and where modified heads have been a little over-zealously ‘relieved’ around the gasket fire-ring line. Especially between the centre two exhaust valves.
What happens here is the excessive torque clamps the fire-ring down so hard it forms a sharp edge that either splits, or gets burnt away by combustion chamber temperatures. Or in the case of the steel-fillet insert type; it twists the fillets out of place. In either case this then blows combustion pressure passed the gasket, pressurising the water jacket. The engine then over-heats or forces water out the front joint between head and block. Uh-hu? Yeah? Makes sense? Happened to you did it? Now you know why. The other scenario is the block and/or head is burnt away between the middle two bores/chambers. Expensive to sort, that.
Alternatively, particularly on standard specifications studs and nuts, increasing the torque value stated will deform the threads on the nut-end of the studs - inverting the thread rather than pulling the head down. This weakens the thread engagement capability, slackening off further when the engine gets hot.
And then there's the problem of pulling the stud thread out of the block. A particular problem when up-rated studs are used, the user believing these super-strong studs will allow higher torque-down values to be used to clamp that head down. Problem is there is not sufficient thread engagement in the block (only a coarse thread remember going into relatively 'soft' cast iron) in a vast majority of cases for the thread engagement to withstand grotesque torque values. 60lb ft is really pushing the holding capability of the standard thread and thread depth. A great deal more thread engagement is needed to cope. But then you just go back to the aforementioned head gasket problems.
The point of using up-rated studs/nuts is to achieve maximum clamping force through quality thread production (maximises engagement area and reduces thread deformation) and reduced material stretch. So the lion's share of the torque applied goes in to clamping the head down, not stretching/deforming the stud.
As a closing note - an example of 'other folk do it differently'…
A very good friend of mine, whom I raced against and have known for many years surprised me a couple of years ago. We were discussing things technical to do with engines. Can't remember how the subject came up - but head nuts torque settings came to the fore. Cutting a long story short - he stated he had only ever used 40lb ft on his engines - and he's been messing about with race A-series motors for longer than I have. He was (is - sorry Steve) a very quick competitor who is definitely always at the sharp end on the field and very, very rarely suffered mechanical failures. None were ever head gasket orientated…makes you think, eh?
Useful part numbers:
C-AHT280 Mini Spares up-rated head stud kit (11 stud suitable)
C-AHT288 'ST' disc-spring washer set
CAM150 A+ long head stud
CAM151 A+ short head stud
TAM1617 A+ short head stud with extra threaded section for coil
CAM4545 A+ head nut with built-in flange/washer
51K1193 Original head nut - more thread engagement than CAM4545
GUG702560HG Latest non-asbestos, steel reinforced, composite head
gasket, large-bore engines
TAM1521B 'Silver-paint' coloured steel reinforced composite head
gasket - forerunner to above
GEG300 Payen copper-composite head gasket, large-bore
C-AHT188 Payen competition copper-composite head gasket, large-bore
- has reinforced fire-rings and larger forward head stud
holes to prevent gasket distortion by head stud 'pull-up'
C-STR1057 Turbo competition steel-reinforced composite head gasket,
large-bore. Only suitable for bores up to +0.040", and
requires special block machine-work to fit
GUG702506HG Payen copper-composite head gasket, small-bore